Economical switching power supply, 220/2x25 volts 3,5 amps. Scheme, description

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Economical switching power supply, 220/2x25 volts 3,5 amps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The power of the power supply is about 180 W, the output voltage is 2x25 V at a load current of 3,5 A. The ripple range at a load current of 3,5 A does not exceed 10% for a ripple frequency of 100 Hz and 2% for a frequency of 27 kHz. The output impedance does not exceed 0,6 ohms. Block dimensions - 170x80x35 mm, weight - 450 g.

The power supply circuit is shown in fig. 5.26.

(click to enlarge)

After rectification by the diode bridge VD1, the mains voltage is filtered by capacitors C1 ... C4. Resistor R1 limits the charging current of the filter capacitors flowing through the rectifier diodes when the unit is turned on. The filtered voltage is fed to a voltage converter built according to the half-bridge inverter circuit on transistors VT1, VT2. The converter is loaded with the primary winding of the transformer T1, which converts the voltage and galvanically isolates the unit output from the AC mains. Capacitors C3 and C4 prevent RF interference from the power supply from entering the network. The half-bridge inverter converts the DC voltage into a square wave AC voltage at a frequency of 27 kHz.

Transformer T1 is designed so that its magnetic circuit is not saturated. The self-oscillating mode of operation is provided by a feedback circuit, the voltage of which is taken from the winding III of the transformer T1 and fed to the winding I of the auxiliary transformer T2. Resistor R4 limits the voltage on winding I of transformer T2. The conversion frequency depends on the resistance of this resistor within certain limits.

To ensure a reliable start of the converter and its stable operation, a start-up unit is used, which is a relaxation generator based on a VT3 transistor operating in an avalanche mode. When the power is turned on through the resistor R5, the capacitor C5 begins to charge and, when the voltage across it reaches 50 ... 70 V, the transistor VT3 opens like an avalanche and the capacitor is discharged. The current pulse opens the transistor VT2 and starts the converter.

Transistors VT1 and VT2 are installed on heat sinks with an area of 50 cm2 each. Diodes VD2...VD5 are also equipped with plate heat sinks. The diodes are clamped between five duralumin plates 40x30 mm each (three middle plates 2 mm thick, two outer ones 3 mm thick). The whole package is tightened with two screws. M3x30, passed through the holes in the plates. To prevent the plates from closing with screws, pieces of a PVC tube are put on them.

Winding characteristics of transformers are given in table. 5.4, the location of the windings of the transformer T1 is shown in fig. 5.27 (the primary winding is wound around the entire perimeter of the ring, not conventionally shown in the figure).

Economical switching power supply, 220/2x25 volts 3,5 amps

Winding wire - PEV-2. To facilitate the start of the converter, the winding III of the transformer T1 should be located in a place not occupied by the winding II. Interwinding insulation in transformers is performed with varnished fabric tape. Between the windings I and II of the transformer T1, the insulation is three-layer, between the remaining windings of the transformers it is single-layer. KT812A transistors can be replaced with KT812B, KT809A, KT704A, KT704V, KD213A diodes - with KD213B. A properly assembled power supply usually does not need to be adjusted, but in some cases it may be necessary to select the VT3 transistor. To check its performance, temporarily turn off the emitter output and connect it to the negative output of the network rectifier. The oscilloscope observes the voltage across the capacitor C5 - a sawtooth signal with a swing of 20 ... 50 V and a frequency of several hertz. If there is no sawtooth voltage, the transistor must be replaced.

The use of this power supply does not eliminate the need to block the power supply circuits of the AF amplifier with large capacitors. Connecting such capacitors further reduces the level of ripple.

Author: Semyan A.P.

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